Unlicensed frequency bands are portions of the radiofrequency spectrum that do not require a license for use and may therefore be used by any device compliant with regulations to transmit or receive radiofrequency signals. Wireless communication devices that transmit or receive signals in licensed or unlicensed frequency bands are typically referred to as nodes, which may include Wi-Fi access points that operate according to IEEE 802.11 standards in the unlicensed spectrum. Nodes also include base stations that operate in the licensed spectrum according to standards such as Long Term Evolution (LTE) standards defined by the Third Generation Partnership Project (3GPP). Base stations that operate according to LTE can implement supplementary downlink (SDL) channels in the unlicensed spectrum to provide additional bandwidth for downlink communications to user equipment that are also communicating with the base station using channels in a licensed frequency band. The licensed frequency bands may be referred to as LTE-L bands and the unlicensed frequency bands may be referred to as LTE-U bands. Base stations may also operate in the unlicensed frequency bands according to Licensed Assisted Access (LAA) standards. Base stations may operate solely in the unlicensed frequency bands without support in licensed frequency bands, e.g., according to emerging standards such as MuLTEFire.
In dense networks, channels in the unlicensed frequency bands can be reused by nodes that operate according to different radio access technologies (RATs) such as Wi-Fi access points and LTE base stations. Communication by the nodes that operate according to the different RATs is coordinated using clear channel assessment techniques to reduce interference between transmissions by the different nodes. For example, listen before talk (LBT) coexistence rules require that each node monitors a channel (e.g., “listens”) to detect energy on the channel prior to transmitting information on the channel. If the detected energy level is below a threshold level, the channel is considered clear and the node is free to transmit on the channel for a predetermined time interval. If the detected energy level is above the threshold level, which indicates that the channel is not clear because another node is transmitting on the channel, the listening node backs off until the energy level falls below the threshold before making another attempt to acquire the channel. The energy detection threshold for Wi-Fi is −62 decibel-milliwatts (dBm) and the energy detection threshold for LTE-U, LAA is −72 dBm, and MuLTEFire is −72 dBm. Wi-Fi nodes may also perform Wi-Fi preamble decoding on signals with detected energy levels below the energy detection threshold and above −82 dBm. The Wi-Fi node backs off if it successfully decodes preambles in transmissions by other Wi-Fi nodes at an energy level between −62 dBm and −82 dBm.